Log cabin structure

ABSTRACT

Log cabin wall element having generally cylindrical surface and a longitudinally extending groove. Groove has spaced side walls extending from groove face to cylindrical outer surface to form groove corners. Radial slot extends inwards from groove face to reduce splitting of log. Initially groove corners of upper element contact upper surface of lower element, groove face being clear of lower element. Weight of element deforms groove corners and brings groove face to contact uppermost portion of lower element, so that weight of upper element is divided between groove corners and groove face. Groove corners and groove face produce pair of space parallel cavities between the first and second elements. Corner deformation can vary along wall element to accommodate differences in fit and improve sealing between adjacent elements.

[ 1 Feb. 4, 1975 LOG CABIN STRUCTURE [76] Inventor: Charles Raymond Fell, PO. Box.

218, Hudson Hope, British Columbia, Canada 22 Filed: Dec.11,l972

21 Appl.No.:3l3,944

[52] U.S. Cl 52/233, 46/20, 46/28 [51] Int. Cl [504]) 1/10 [58) Field of Search 52/233; 46/20, 28

[56] References Cited UNITED STATES PATENTS l,287,77l l2/l9l8 Schmitt 46/20 2,040,1l0 5/1936 Tahvonen t 52/233 2,l30,23l 9/]938 Forclea v t .r 52/233 2,525,659 l0/l950 Edson 52/233 3,l39,950 6/1965 Johnson 52/233 FOREIGN PATENTS OR APPLICATIONS 783,292 4/l968 Canada 52/233 l29,688 4/l946 Sweden t t r r 52/233 41,122 3/l924 Norway 4 t H 52/233 Primary Examiner-Ernest R. Purser Assistant Examinerhlenry Raduazo Attorney. Agent, or Firm-Carver and Company [57] ABSTRACT Log cabin wall element having generally cylindrical surface and a longitudinally extending groove. Groove has spaced side walls extending from groove face to cylindrical outer surface to form groove corners. Radial slot extends inwards from groove face to reduce splitting of log. Initially groove corners of upper element contact upper surface of lower element, groove face being clear of lower element. Weight of element deforms groove corners and brings groove face to contact uppermost portion of lower element. so that weight of upper element is divided between groove corners and groove face. Groove corners and groove face produce pair of space parallel cavities between the first and second elements. Corner deformation can vary along wall element to accommodate differences in fit and improve sealing between adjacent elements.

10 Claims, 7 Drawing Figures PATENTED FEfl 41975 SHEEI 10F 2 LOG CABIN STRUCTURE BACKGROUND OF THE INVENTION I. Field of the Invention This invention relates to a wall element for use in log cabin and similar buildings.

2. Prior Art Traditionally log cabins were made of logs cut to length, the logs having a saddle cut near each'end so that, when assembled, longitudinal peripheral contact between adjacent logs is approached. In practice, irregularities between the logs and taper cause gaps between adjacent logs and produce ill-fitting corners.

Commonly modern log cabins are constructed of logs cut to suitable length and turned to cylindrical form so that they have zero taper. Sometimes such turned logs have axial grooves extending lengthwise, the grooves accepting location members to locate and seal one log relative to an adjacent log. Such grooving requires accurate machining and close tolerances are required to obtain a satisfactory fit. Swelling and checking often cause difficulties. Radial splitting as the log dries is reduced by providing narrow radial slots extending into the log.

SUMMARY OF THE INVENTION The present invention reduces difficulties of the prior art by providing a wall element for use in log cabins in which satisfactory seal between adjacent wall elements is attained without requiring accurate machining of location members. Radial splitting is also reduced.

A wall element according to the present invention has a central longitudinal axis and a generally cylindrical surface concentric with the axis. The cylindrical surface has a longitudinally extending groove having a groove face. the groove having spaced groove side walls extending from the groove face to the cylindrical surface to form groove corners, the face and side walls being within planes parallel to the central longitudinal axis. When a similar second wall element is disposed beneath a first wall element, the groove face of the first wall element lies on an upper surface of the second element, the groove corners being forced against the lower element and deformed to lock the wall elements together to produce two longitudinally extending cavities between the first and second wall elements. The corners produce effective sealing between the elements. and deformation of the groove corners varies along the wall elements and accommodates variations in fit between elements.

A detailed disclosure following related to the drawings describes one embodiment of the invention, which however is capable of expression in structure other than that particularly described and illustrated.

DESCRIPTION OF THE DRAWINGS FIG. I is fragmented perspective of a corner ofa log cabin using wall elements according to the invention,

FIG. 2 is a simplified fragmented vertical section through a portion of a wall of the cabin remote from a corner,

FIG. 3 is a simplified fragmented detail vertical section of adjacent wall elements, crosshatching being omitted,

FIG. 4 is a fragmented central section of portions of wall elements adjacent a corner,

FIG. 5 is a simplified section on 5-5 of FIG. 4, FIG. 6 is simplified bottom plan view of an element of FIG. 4,

FIG. 7 is a diagram showing dimensional characteristics of the wall element.

DETAILED DISCLOSURE FIGS. 1 and 2 A portion l0 of a log cabin has a corner II formed at an intersection of walls made of wall elements according to the invention. A lower portion of one wall has a wall element 12 cut essentially along a horizontal diameter so as to lie as shown on foundations of the building (not shown), as is common practice with log cabins of this construction. With reference to FIG. 2 a log cabin wall element 13 according to the invention has a central longitudinal axis 14 and is disposed upon a similar second wall element 16. The element 13 has a generally cylindrical surface [5 having a radius. the surface being concentric with the longitudinal axis. A lower portion of the cylindrical surface of the element 13 has a longitudinal groove 17, the groove extending along the element. A radial slot 18 extends from the groove to an end edge 20, radial length of the slot being approximately one-third of the radius of the element IS, the slot similarly extending along the element. FIG. 3

The groove 17 has a generally plane groove face 22 having spaced parallel generally plane groove side walls 24 and 25, the side walls extending from the groove face to the cylindrical surface to form groove corners 26 and 27. The groove has a groove width 28 defined as space between the groove corners 26 and 27. The groove face and side walls are within planes parallel to the axis 14, the sidewalls being normal to the groove face 22.

.The radial slot 18 is disposed symmetrically about a central plane or centreline 32 passing through the axis 14, not shown, the slot having a width 36 equal to kerf of a saw used to cut the slot, about one-quarter of an inch. The slot is provided to reduce splitting ofthe element as the wood dries and for the following discussion effects of the slot or splitting are ignored. The longitudinally extending groove 17 is thus defined by the groove face 22 disposed generally normally to the longitudinal plane 32 and two spaced groove sidewalls 24 and 25 extending normally to the groove face 22 and generally parallel to the longitudinal plane 32 to define a generally rectangular cross-sectioned groove. Due to the slot above the groove, the groove corners have the capacity to resiliently spread laterally.

The cylindrical surface l5 ofthe element is produced as a virtual are shown as a broken line 29, the are being concentric with the cylindrical surface 15 and having a radius equal to the radius of the cylindrical surface. A virtual depth 30 of the groove is defined by a radial distance between a mid point 3] of the groove face 22 and the virtual are 29, thus is depth of the groove measured on the centre line 32. The virtual depth represents a maximum depth of cut a cutter or planer would make to cut the groove 17 in a cylindrical wall element having a surface continuing as the virtual are 29. The groove 17 has an initial true depth 34 defined as perpendicular distance from the corner 27 to the face 22, and by geometry is one half of the virtual depth 30.

An upper cylindrical surface 38 of the second wall element 16 is shown as a broken line 38.I in a first posilion in contact with the corners 26 and 27, and in full outline in a second position in contact with the groove face 22. initial true depth of the groove is such that, in the first position the surface 38 clears the face 22 by a clearance 39. Weight of the wall element 13 and structure above the element compresses and deforms the corners 26 and 27 and portions of the surface 38 con tacted by the corners so that the corners engage the surface. The deformation permits the upper surface 38 to approach the groove face 22 until an equilibrium condition is attained in which weight of the element l3 and structure above is distributed about the two corners 26 and 27 and a central portion of the face 22 ad jacent the centreline 32. Compression of the corners 26 and 27 reduces the initial true depth of the groove to a final true depth 40, difference between the depths 34 and 40 being approximately equal to the clearance 39. The final true depth is generally between one-sixteenth and one-eighth of an inch less than the initial true depth and is dependent on physical characteristics and diameter of the wall elements. With respect to FIG. 3, the distance between the horizontal planar supporting surface or groove face 22 of the groove 17 and the uppermost cylindrical portion of the cylindrical surface 38 has an initial clearance of one-sixteenth to one-eighth of an inch prior to seating ofthe groove corners and the horizontal planar supporting surface with the cylindrical surface of th lower or second wall element. It would appear that more weight from upper wall elements is transferred through the groove face than through the groove corners. The corners 26 and 27 sink into the surface 38 and tend to locate the corner relative to the lower element thus reducing a tendency of groove 17 and the slot 18 to open, and thus serve as a locking means. A substantially constant groove width along the elements is maintained. The sidewalls 24 and 25 are parallel to each other, seen in FIG. 3, and extend normally from the groove face 22. The sidewalls can be inclined at an angle to the groove face 22 somewhat greater than a right angle, but this tends to reduce sinking of the corner into the surface 38 and tends to open the groove 17.

Variations in loading between adjacent wall elements and dimensional variations between elements results in some positions of corner deformation being in excess of one-eighth of an inch, whereas in other positions on the same wall element corner deformation may be negligible. Such fluctuations in corner deformation provide a convenient means of accommodating the variations to reduce chances of gaps occuring between adjacent elements, thus producing an essentially weatherproof seal between the groove corners and the lower wall element. Also, as the groove corners are below and straddle an uppermost portion ofa lower element, horizontal lateral movement between adjacent elements is essentially eliminated without using locating members as in prior art construction. This is of particular advantage with small diameter elements, which are less stiff than larger diameter elements. If desired to compensate for deformation of the corners 26 and 27, the groove width 28 can be adjusted to be somewhat less than that required with no corner deformation. Alternately the groove depth can be similarly compensated.

As can be seen, a portion of the groove face 22, the groove sidewall 24 and the portion of the upper surface 38 of the second element between the groove corner and the groove face define a cavity 42 extending longitudinally between the first and second wall elements. A similar cavity 43 is also defined by a portion of the groove face 22, the sidewall 25 and a portion of the upper surface 38 and thus two essentially similar cavities extend in spaced parallel relationship between the first and second elements. Before the element I3 is positioned on the element 16, a strip of compressible insulation (not shown) can be fitted in the groove 17, which insulation, when the element l3 rests on the element 16 is compressed and fills the cavities 42 and 43, and also provides resilient mounting between the wall elements.

As seen, overall thickness of the wall at a join between adjacent elements is determined by groove width and depth, and wall element size. Particularly for small diameter wall elements with relatively wide grooves. overall wall thickness at a join is reduced a relatively small amount from wall thickness remote from the join, which thickness is maximum at a diameter of the wall element. Thus heat losses by conduction through the join are only slightly more than heat losses through the element remote from the join. Heat losses through the join are reduced further by the compressible thermal insulation.

FIGS. 4, 5 and 6 An end face 49 of the element 13 of one wall is adjacent the right-angled corner I] of the cabin (FIG. 1). and has a saddle cut 50 to accommodate a lower element (not shown) in an adjacent wall, the second wall, the walls being inclined at a corner angle. The second wall has an element 51 disposed directly above and supported in part by the lower element.

The saddle cut 50 has a concave, partially cylindrical saddle face 52 concentric with a saddle axis 54, which axis is inclined at a saddle angle 55 to the axis 14 of the element 13. the saddle angle being complementary to the corner angle, i.e., For walls intersecting at a corner angle other than 90, the saddle angles of the elements are equal to each other and complementary to the corner angle, an alternative saddle angle 62 on an element having a longitudinal axis 64 being shown in FIG. 6 only. The saddle face 52 has a radius 56 approximately equal to the radius of the cylindrical surface of the log to be accommodated in the saddle cut, and thus is approximately equal to the radius of the cylindrical surface 15. The saddle has a thickness 57 defined as a radial distance between a midpoint 59 of the saddle face and an uppermost point 61, hereinafter termed outermost point, on the cylindrical surface of the wall element, the saddle thickness being measured within the plane 32.

In common log cabin construction the saddle thickness as defined above is approximately equal to the radius of the logs at the corner, assuming a cylindrical log. in the present invention the saddle thickness is reduced by an amount proportional to the depth of the groove 17, as will be described with reference to FIG. 7.

At a corner using staggered wall elements the wall element 13 is disposed beneath the wall element 51, the wall element 51 having a similar saddle face 63, FIG. 5 only, which lies on the upper surface of the wall element 13. Thus the saddle face ofa first wall element engages an upper surface of a second wall element of an adjacent wall, the second element staggered below the first element, the first element being engaged by a saddle face of a third wall element of the adjacent wall staggered above the first wall element.

FIG. 7

As previously stated, for a common saddle cut the saddle thickness is approximately equal to the radius of the cylindrical wall element. that is half the diameter. in the present invention the groove 17 (FIG. 3) modi fies saddle thickness by an amount dependent of groove depth as is described below. in the following idealised analysis. the groove face 22 (FIG. 3) is considered as a datum face as this face contacts a lower element. As deformation of groove corners tends to vary considerably in distance from the groove face. groove corners are considered to be unpractical for use as a datum.

Consider first and second wall elements A and B of a first wall. and a third wall element C of a second wall which intersects the first wall at right angles.

Let diameter of the cylindrical surfaces of the wall elements D and virtual depth 30 of groove d Therefore effective depth of wall member i.e., vertical distance between uppermost surface and groove face D d An effective central plane Ec of wall member C is midway between uppermost surface Uc of C and groove face 6c of C. For staggered wall members of adjacent walls, as shown in FIGS. 4 and 5, Be is coplanar with a join between adjacent wall elements A and B. i.e.. Ec is coplanar with groove surface Ga of A.

Thus Be is spaced equally from both Uc and Ge, a distance D d/2 Geometric true centre Ta of the cylindrical surface of wall member A is on the central longitudinal axis (14. FIG. 2).

Distance between Ta and Ga 0/2 d Because all wall elements are equal,

Distance between Ea and Ga =D d/Z Therefore spacing Sa between effective centre Ea and true centre Ta (D d)/2 (D/2 d) d/2 Thus effective centre Ea is one-half virtual groove depth d spaced above true centre Ta. Thus saddle thickness is one-half of groove virtual depth less than the radius of the surface 15. in contrast to a saddle depth of the radius in prior art construction.

The above is an idealised consideration and in practice is used as a starting point for a trial and error cutting of the saddle cut. depth of which may be compensated somewhat.

Wall Element Dimensions A suitable wall element can be made from grooved and slotted cylindrically turned log having a diameter of between 6 and I2 inches. a particular wall usually having logs of the same diameter. A lower limit is about four inches diameter. an upper limit for practical handling and economics being about 16 inches diameter. As larger wall elements have a shallower curved upper surface. for a constant groove width. groove depths of larger elements are generally smaller than groove depths of sm sller elements. so as to maintain contact of groove face with an uppermost surface of a lower element. A simple geometrical construction provides a starting point to determine groove virtual depth. simple trial and error methods determining actual depth of cut to be made. Virtual depth i.e.. depth of cut is considered in the following.

For a 12 inch diameter log. a five-sixteenths groove depth has been found convenient with a groove having a width of 2% inches. That is, groove depth is 5 percent of the radius. and groove width is 42 percent of the radius. For a 6 inch diameter log with a 2% inch groove width, a groove depth of eleven-sixteenths of an inch has been found suitable. That is groove depth is 23 percent of the radius and groove width is 84 percent of the radius.

Outer practical limits ofgroove depth might be about a l inch groove depth on a 4 inch diameter log. that is percent of the radius. and a three-sixteenths groove depth on a 16 inch diameter log. that is 2 percent of the radius. Outer practical limits of groove width might be a 3 inch groove width on a 4 inch diameter log. that is I50 percent of the radius. and a 2 inch groove width on a 16 inch diameter log. that is 25 percent of the radius. As can be seen groove width and depth are interdependent for a fixed log diameter. and a suitable combination is found by the graphical construction or trial and error. Simple measurement of the initial true depth of cut 34 FIG. 3, before corner compression, is half the virtual depth. A generalized relationship is that the depth of the groove decreases with increase in diameter of the wall elements so as to attain sufficient corner compression to permit the planar supporting surface to absorb most of the weight transmitted through the first wall element.

It is usual to select and fix two variables. namely log diameter and groove width. the groove width being determined by a cutter used to produce the groove. The groove depth is adjusted accordingly to attain sufficient corner compression so that most of the weight of the log is transferred through the groove face to an upper surface of the lower log.

The groove, slot and saddle cut above can be produced in a cylindrical log by a variety of apparatus. Particular apparatus for turning. grooving and slotting a log is described in a patent application of the present inventor entitled Apparatus for Machining Logs. Apparatus for cutting a saddle cut in a log is described in a patent application of the present inventor entitled "Apparatus for Grooving Logs." both applications above and the present application being filed concurrently.

I claim:

1. A portion ofa log cabin wall structure. comprising:

a. an elongated first wall element having a substantially cylindrical cross section.

b. an elongated second wall element having a substantially cylindrical cross section and disposed immediately below and in alignment with the first wall element.

c. locking means disposed along the lowermost surface of the first wall element for firmly engaging and holding the first wall element in superposed relation with the second wall element.

d. the locking means including a recessed horizontal planar supporting surface for directly supporting the first wall element which has two spaced substantially perpendicular side walls. the planar supporting surface directly engaging and supported on the uppermost peripheral section of the cylindrical surface of the second wall element.

e. the locking means also including laterally spaced groove corners formed at the intersection of the spaced side walls with the cylindrical surface of the first wall element, said groove corners disposed below the horizontal planar supporting surface and which are resiliently urged into sinking engagement with the cylindrical surface of the second wall element,

. a longitudinally extending radial slot disposed perpendicular to the horizontal planar supporting surface and having a length approximately one-third the radius of the wall element so as to permit resilient lateral spreading of the groove corners,

. the distance between the groove corners before engagement with the cylindrical surface ofthe second wall element being slightly less than the cordal distance between the two opposed points of engagement with the laterally spread groove corners after the horizontal planar supporting surface comes into engagement with the cylindrical surface of the second wall element.

The portion of a log cabin wall structure as set forth in claim 1, wherein:

forth in claim 1, wherein:

the depth of the groove formed by the planar horizontal surface decreases with increase in the diameter of the first and second wall elements so as to attain sufficient corner compression to permit the planar supporting surface to absorb most of the weight transmitted through the first wall element.

4. The portion of a log cabin wall structure as set forth in claim 3, wherein:

the groove has a virtual depth defined by a radial distance between a midpoint of the groove face and the cylindrical surface of the wall member produced as a virtual are extending across the groove, the virtual are being concentric with the cylindrical surface of the wall element and having a radius equal to the radius of a cylindrical surface.

b. the initial groove depth being the perpendicular distance from the groove corner to the horizontal planar supporting surface and having a value one half that of the virtual depth.

5. The portion of a log cabin wall structure as set forth in claim 3, wherein:

the first and second wall elements have a diameter of approximately twelve inches,

b. the groove depth is approximately five-sixteenths of an inch,

c. the groove width is approximately 2 inches. 6. The portion of a log cabin wall structure as set forth in claim 3. wherein:

a. the diameter of the first and second wall elements is approximately 6 inches,

the groove depth is approximately elevensixteenths of an inch,

c. the groove width is approximately 2 Va inches.

7. The portion of a log cabin wall structure as set forth in claim 3, wherein:

a. the diameter of the first and second wall elements is approximately 4 inches,

b. the groove depth is approximately I 5% inches,

c. the groove width is approximately 3 inches.

8. The portion of a log cabin wall structure as set forth in claim 3, wherein:

a. the diameter of the first and second wall elements is approximately 16 inches,

b. the groove depth is approximately three-sixteenths of an inch,

c. the groove width is approximately 2 inches.

9. The portion of a log cabin wall structure as set forth in claim I, wherein:

a. the two cavities formed between the horizontal planar surface and the cylindrical surface of the second wall element are filled with insulation.

10. The portion of a log cabin wall structure as set forth in claim l, wherein:

a. the first wall element includes a first saddle having a concave, partially cylindrical first saddle face concentric with the first saddle axis, which axis is inclined at a saddle angle to the longitudinal axis of the wall element,

b. the second wall element has an essentially similar saddle to the first wall element, the first and second wall elements being a portion of a first wall, and further including a portion of a second wall intersecting and inclined at a corner angle to the first wall,

c. the second wall having an essentially similar third wall element having a similar saddle and saddle face, the third wall element disposed between and intersecting the first and second wall elements at the corner angle, so that the saddle face ofthe third wall element engages the upper surface of the second element and the saddle face of the first wall element engages the upper surface of the third element, the saddle angles being complementary to the corner angle.

It I! i t l 

1. A portion of a log cabin wall structure, comprising: a. an elongated first wall element having a substantially cylindrical cross section, b. an elongated second wall element having a substantially cylindrical cross section and disposed immediately below and in alignment with the first wall element, c. locking means disposed along the lowermost surface of the first wall element for firmly engaging and holding the first wall element in superposed relation with the second wall element, d. the locking means including a recessed horizontal planar supporting surface for directly supporting the first wall element which has two spaced substantially perpendicular side walls, the planar supporting surface directly engaging and supported on the uppermost peripheral section of the cylindrical surface of the second wall element, e. the locking means also including laterally spaced groove corners formed at the intersection of the spaced side walls with the cylindrical surface of the first wall element, said groove corners disposed below the horizontal planar supporting surface and which are resiliently urged into sinking engagement with the cylindrical surface of the second wall element, f. a longitudinally extending radial slot disposed perpendicular to the horizontal planar supporting surface and having a length approximately one-third the radius of the wall element so as to permit resilient lateral spreading of the groove corners, g. the distance between the groove corners before engagement with the cylindrical surface of the second wall element being slightly less than the cordal distance between the two opposed points of engagement with the laterally spread groove corners after the horizontal planar supporting surface comes into engagement with the cylindrical surface of the second wall element.
 2. The portion of a log cabin wall structure as set forth in claim 1, wherein: a. there is a clearance of approximately one-sixteenth to one-eighth of an inch between the horizontal planar supporting surface of the first wall element and the upper periphery of the cylindrical surface of the second wall element on initial engagement and before full seating of the horizontal planar supporting surface on the cylindrical surface of the second wall element.
 3. The portion of a log cabin wall structure as set forth in claim 1, wherein: a. the depth of the groove formed by the planar horizontal surface decreases with increase in the diameter of the first and second wall elements so as to attain sufficient corner compression to permit the planar supporting surface to absorb most of the weight transmitted through the first wall element.
 4. The portion of a log cabin wall structure as set forth in claim 3, wherein: a. the groove has a virtual depth defined by a radial distance between a midpoint of the groove face and the cylindrical surface of the wall member produced as a virtual arc extending across the groove, the virtual arc being concentric with the cylindrical surface of the wall element and having a radius equal to the radius of a cylindrical surface, b. the initial groove depth being the perpendicular distance from the groove corner to the horizontal planar supporting surface and having a value one half that Of the virtual depth.
 5. The portion of a log cabin wall structure as set forth in claim 3, wherein: a. the first and second wall elements have a diameter of approximately twelve inches, b. the groove depth is approximately five-sixteenths of an inch, c. the groove width is approximately 2 1/2 inches.
 6. The portion of a log cabin wall structure as set forth in claim 3, wherein: a. the diameter of the first and second wall elements is approximately 6 inches, b. the groove depth is approximately eleven-sixteenths of an inch, c. the groove width is approximately 2 1/2 inches.
 7. The portion of a log cabin wall structure as set forth in claim 3, wherein: a. the diameter of the first and second wall elements is approximately 4 inches, b. the groove depth is approximately 1 1/2 inches, c. the groove width is approximately 3 inches.
 8. The portion of a log cabin wall structure as set forth in claim 3, wherein: a. the diameter of the first and second wall elements is approximately 16 inches, b. the groove depth is approximately three-sixteenths of an inch, c. the groove width is approximately 2 inches.
 9. The portion of a log cabin wall structure as set forth in claim 1, wherein: a. the two cavities formed between the horizontal planar surface and the cylindrical surface of the second wall element are filled with insulation.
 10. The portion of a log cabin wall structure as set forth in claim 1, wherein: a. the first wall element includes a first saddle having a concave, partially cylindrical first saddle face concentric with the first saddle axis, which axis is inclined at a saddle angle to the longitudinal axis of the wall element, b. the second wall element has an essentially similar saddle to the first wall element, the first and second wall elements being a portion of a first wall, and further including a portion of a second wall intersecting and inclined at a corner angle to the first wall, c. the second wall having an essentially similar third wall element having a similar saddle and saddle face, the third wall element disposed between and intersecting the first and second wall elements at the corner angle, so that the saddle face of the third wall element engages the upper surface of the second element and the saddle face of the first wall element engages the upper surface of the third element, the saddle angles being complementary to the corner angle. 